1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to a thin, two die semiconductor package and method of fabrication thereof.
2. State of the Art
Semiconductor dice are usually packaged to provide a protective structure therefor and to facilitate mechanical and electrical connection thereof to a higher-level assembly, such as a carrier substrate in the form of, for example, a module board or a motherboard. A conventional approach to packaging one or more semiconductor dice is to secure the die or dice to a metal lead frame, wire bond the die bond pads to lead fingers of the lead frame, and encapsulate the resulting assembly with a transfer-molded, silicon-filled thermoplastic polymer with lead fingers of the lead frame extending to the exterior of the molded package. A wide variety of such packages have been developed over the years.
One significant consideration in encapsulation of die assemblies has always been the reliability of the encapsulated assembly. Voids in the encapsulant volume may lead to environmental contamination and failure of the package, or even rupture thereof under high temperature service. Mismatches in the coefficient of thermal expansion (CTE) between the die, the metal lead frame and the encapsulant contribute to internal stresses which can jeopardize package integrity. Further, substantial mismatches in the volume of encapsulant on opposing sides of a lead frame lead to unbalanced stresses within the package as the encapsulant cools to a solid state after molding, resulting in a tendency of the package to “bow” to one side under bending stress and enhancing the potential for stress cracking as well as delamination of the encapsulant from lead fingers extending therethrough.
The potential for the above-noted problems may be aggravated when multiple semiconductor dice are packaged together. For example, when two semiconductor dice are to be packaged in a transfer molded casing, placing both dice on the same side of a lead frame die paddle may, again, exacerbate stresses within the package, due to the unbalanced masses of encapsulant as well as CTE mismatch among the package components.
One conventional approach to avoid this problem is to back-bond each of the two semiconductor dice to the lead frame paddle to provide symmetry on both sides of the lead frame. However, this requires inverting the lead frame paddle for mounting and wire bonding of the second die. Another approach to packaging multiple dice involves the use of a downset lead frame die paddle to facilitate central placement of stacked dice facing in the same direction (active surface up) for ease of wire bonding within the package, but this requires an additional step in deforming the lead frame and the use of differently sized dice to provide access to the bond pads of each die. Yet another packaging approach includes placing two semiconductor dice facing in the same direction (active surface up) on opposing sides of a lead frame, the lower die having a central row of bond pads and the upper die having peripheral bond pads. However, this requires the use of a standoff element under the upper die to provide adequate clearance for the wire bond loops connecting the lower die to the lead frame, thus demanding the use of two differently configured dice and an extra assembly step to place the standoff element, and results in an undesirably thick package.
It would be desirable to provide a lead frame-based, transfer molded package configuration for two semiconductor dice which may incorporate two identically sized dice having the same or similar bond pad arrangements thereon, wherein wire bonding may be effected without inversion of the assembly, and which results in a thin, substantially symmetrical package.